1. Field of the Invention
The invention is directed to a low-level voltage clamping circuit. More particularly, the low-level voltage clamping circuit of this invention does not require its own separate power supply, but rather drives its active components from the fault voltage of the circuit being protected.
2. Description of the Prior Art:
Certain electrochemical processes require low level excitation voltages in order to function. By low level voltages, what is meant is about one volt or less. In these electrochemical processes, if for any reason the low-level voltages were to experience even a partial increase above the normal excitation range, chemically irreversible process reactions might begin to occur. Accordingly, a protective low-level voltage clamp is required to limit the maximum applied excitation voltage to an acceptable level. Furthermore, such a protective clamp must function under all circumstances, whether the conditions are normal or fault.
It has been suggested that operational amplifier clamping circuits be employed because at or about 1.0 volt DC, such clamping circuits are very effective. Excellent off-on characteristics as well as sharp switching at the pre-set clamping level are exhibited. However, if the dual differential power supply of the operational amplifier fails, the clamp is lost. Moreover, if only one side of the dual differential power supply fails, not only is accurate clamping lost, but the clamping circuit itself may now act as a source of voltage well in excess of the required clamping level. While operational amplifiers that can operate from a single-sided power supply at as low as 1.0 volt DC are available, if the power supply fails, the clamp is lost. Furthermore, even though single-sided operational amplifiers can be incorporated into a two-terminal protective network powered only by the two terminals at which clamping must be effected, the voltage normally available to power the protective network is lower than the minimum voltage required to power such an operational amplifier. As a result, the operational amplifier may actually act as a source or sink for the excitation voltage to the electrochemical process rather than acting as a high off impedance network.
It is also known to use two-terminal clamping devices such as Zener diodes or integrated circuit reference diodes. The diodes offer a relatively high off impedance below their breakover voltage and they do not require a power supply in order to clamp voltages. There are, however, critical deficiencies in such devices relative to the application of the clamping circuit of this invention. The aforementioned Zener diodes and reference diodes either are not available in the low voltage range of interest, do not provide sharp switching knees or cannot sink high levels of fault current.
It is, therefore, an object of this invention to provide a circuit design for a low-level voltage clamp that drives its active components from the fault voltage of the protected circuit.
It is also an object of this invention to provide a low-level voltage clamping circuit which can operate in a range from its clamping level at one end down to zero volts at the other.
It is a further object of this invention to utilize within the circuit, functional blocks comprised of discrete semiconductor components which enable the invention to operate in the range of between 1.0 to 0 volts DC.
It is still an object of the invention to circumvent power limitations on the amount of fault current the clamp can handle by employing an intermediate amplifier stage and current sinking output stage.